Re: Variable Power Supply Circuit Design Question
From: Tim Wescott (tim_at_wescottnospamdesign.com)
Date: 09/19/04
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Date: Sun, 19 Sep 2004 15:39:18 -0700
Mook Johnson wrote:
> For us to be of much help, you either need to describe the output
> requirements in terms of voltage range and current limit range or describe
> the type of load it will be seeing (4000 series CMOS, 74HC, analog, etc.)
>
> You can add current limiting directly to the LM317 without having the 7805
> (see lm317 datsheet). Make sure you use a heatsink large enough to handle
> the power under a short curcuit situation. The 7805 and the output diode
> will add voltage drop that will be dependent on the load (output impedance)
> and cause errors in your output voltage setting.
>
> LM317 datasheet probebly has what you need.
>
>
-- snip --
That's stated very mildly. Let me amplify: The circuit behavior with
the 7805 as a series-pass element will _not_ be satisfactory. It's
probably great for charging batteries, there may even be some parasitic
variations in the charge voltage with temperature due to the 7805, but
it is _not_ what you want in a power supply for logic.
Either use Mook's suggestion for current limiting with the 317, or use a
circuit based on the 723 (I think they're still available).
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com
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